I too am on 1 and 1 (I even made the horrible mistake of calling their tech support... ugh), and making the change as Brian <THANKS BRIAN!!!!> indicated DID help me to install one more plugin... but the next one, BAM, same error. Trying memory=30MB, or even 128, didn't help either.

Assuming that this is an issue with memory allocation to php on the server, is there any way to reliably increase the memory allocation so I can run an insane amount of plugins?

Here's what presently WORKS (but I wanted a few more, like GD-Star Rating, Subscribe2, WP-Polls... you know, irrelevant but fun things):

this is a 1and1 problem
I had to endure their crap for to long I dont like calling to the other side of the world where they have NO CLUE
I can wait to move to another host
AMEN to the brother with the
AddType x-mapp-php5 .php
added that as I was on my 2nd call waiting on hold
check the plugin page BAMN
You da man
thanks